How birds hear

The ears are connected by an air-filled chamber below the brain case.
The most sensitive region of the birds hearing is in the 1 to 3 kilohertz range 
also the peak acoustic noise output of a modern jet engine. The intense jet noise
may overstimulate the birds inner ear sensory receptors, interfering with a bird's
ability to hear, and leaving them unaware they may be flying directly into an aircraft.